Belgian prosecutors have begun one of the country's biggest-ever terrorism trial, accusing 46 men of being members of a terrorist organization, Sharia4Belgium, that indoctrinated young men to fight in Syria.

Nine of those charged were present in an Antwerp court on Monday, including alleged leader Fouad Belkacem. Others are thought to be fighting in Syria, with many possibly having died in the country's civil war.

The case centers on Sharia4Belgium and its efforts to indoctrinate young Muslims through social media and readings, and send them to Syria to fight.

The prosecutor cited parents of some of the young Belgian jihadists as saying their children would never have thought of fighting overseas if they had not been brainwashed by Sharia4Belgium.

The maximum penalty for leading a terrorist group in Belgium is 20 years in prison.

Authorities worldwide fear those traveling to Syria to fight will return home battle-hardened and trained to carry out terrorist attacks.

The United Nations Security Council last week unanimously passed a resolution cracking down on the flow of foreign fighters to militant organizations such as the Islamic State group.

The council voted 15-0 Wednesday to compel countries to make it a crime for their citizens to travel abroad to fight with militants or recruit other people to do it.